1. Field of the Invention
This invention relates to a position control system for moving a certain movable member to a selected position for positioning and more particularly, to an apparatus for moving a head member such as, for example, the data-transducer of a disk drive or the printing head of a printer for positioning and their methods.
2. Description of the Prior Art
Recently, enhancement of performances of information equipment is surprising and accompanied with which, a driving means used for the data-transducer of a disk drive or the printing head of a printer has been required to provide with a position control system operable at a higher speed and accuracy as well as a finer resolution.
Conventional driving means for use in such information equipment have frequently used an open-loop position control system utilizing a stepping motor.
Hybrid-Permanent-Magnet type of stepping motor has been widely used for this purpose. This type of motor has a rotor consisting of a permanent magnet magnetized in the direction of rotating axis and a couple of magnetic materials mounted on both sides of the permanent magnet. These magnetic materials are respectively provided with arrays of magnetic pole-teeth engraved at constant pitches in the circumferential peripheries. On the other hand, the motor of this type has a stator consisting of a magnetic material core having groups of magnetic pole-teeth on its surface in confronting relation to the rotor and a plurality of coils provided in the slots of the magnetic material core.
Current-feeding circuit for the stepping motor of hybrid-permanent-magnet type includes bridge-circuits respectively structured of a plurality of semiconductor switches and these coils are connected between the middle points of these bridge-circuits. So connected that if these switches are successively switched, currents can be alternatively flowed in these coils in order. As a result, the rotor changes the position to rotate due to the series of magnetization of these coils. With the most typical 2-phase feeding stepping motor, the rotor rotates by 1/4-teeth pitch (90.degree. in terms of electric angle) by synchronizing with an inputted stepping pulse signal.
The usual open-loop position control system utilizing the stepping motor shown above is well-known in U.S. Pat. No. 4,568,988; "Position Control System of a Disk System". Such an open-loop position control system offers the advantage of using electronic control circuit simple in structure. Input signals to the electronic circuit are the stepping pulse signal and a command signal for the moving direction. The outputs from the electronic control circuit are the feeding-currents. The electronic control circuit consists mainly of the above-mentioned bridge-circuits and a sequence circuit for switching the semiconductor switches in response to the above-mentioned input signals. The usual open-loop position control system supplies currents to the coils of the stepping motor through the bridge-circuits and the sequence circuit in response to these input signals to rotate the rotor. As a result, a head member such as, for example, the data-transducer of a disk drive or the printing head of a printer can be moved for positioning. The term "open-loop" means that the moving distance of head member can be determined by the number of stepping pulses of an input signal only.
Such a conventional position control system as utilizes a stepping motor as a driving means makes possible that by the number of stepping pulse signals and the command signal for moving direction, for example, the head member can be moved for positioning, so that the control can be easily achieved by means of a micro-processor. In addition, the system offers the higher effectiveness of power as compared to a system utilizing a driving means such as, for example, the voice coil motor or servo-motor. This is because a power device such as the stepping motor having tooth-shaped magnetic pole-teeth in the magnetic circuit is superior in the magnetic effectiveness and an extremely large torque can be generated even by a small current. As a result, this system was characterized by being less in power consumption.
However, some problems have been pointed out on the speed and accuracy of this system. A case in point is that when the stepping motor as a driving means is to be stopped for positioning, it will become vibrative around a standstill position. This means that it may require a considerable settling time for positioning. In order to decrease the vibration thereby to reduce the settling time, the rotor or movable members can be provided with a mechanical viscous damping. However, this may result in a complex structure or may cause the high speed operation to be prevented due to this viscous damping, which have been pointed out for this system as disadvantages such as not to be pointed out for that utilizing a voice coil motor or servo-motor.
Also, the stepping motor can obtain a speed synchronized with the stepping pulse signal. As a result, in order to move a moving member at a higher speed, the frequency of a stepping pulse signal can be increased on a theoretical basis. When drived at a higher speed, however, differing from the case when drived at a lower speed with a large torque, because of the effect of a time constant or magnetic hysterisis loss that a coil possesses, the rising of a current will be delayed, and the torque can not be developed effectively, resulting in the generation of a small torque. In this case, to attempt to unreasonably enhance the frequency of the stepping pulse signal will easily cause an out of stepping to take place.
A position control system having the timing of a stepping pulse signal controlled by providing sensor elements on the moving member of the stepping motor in order to prevent the out of stepping has been disclosed in U.S. Pat. No. 4,044,881 as an example showing the application for a serial printer. This system senses the presence of the magnetic pole-teeth to produce a pulse signal and then, controls the timing of the generation of a stepping pulse signal in response to this pulse signal thereby to prevent the out of stepping. However, this method can do the timing control only, so that when, for example, a vibration or impact is applied from the outside, it is difficult to prevent such a disturbance.
In addition, if the magnetic pole-teeth pitch of each of the rotor and stator is made small to increase the number of teeth, one step angle can be decreased, so that the resolution of positioning of a moving member is ought to be increased. However, in practice, not only there exists a limitation upon the mechanical accuracy, but also the switching frequency of the current of each coil is increased, so that the effectiveness can be rapidly decreased. This is explained in detail in "Parameters Governing the Dynamic Performance of Permanent-Magnet Stepping Motors" (A, Hughes Proc. Sixth Annual Symposium of Incremental Motion Control System and Devices, 1977, pp. 39-47). In addition, in order to improve the resolution of positioning, if the magnetic pole-teeth pitches of the rotor and stator are made small to increase the number of them thereby to decrease one step angle, there exists such a trend that the stiffness and position holding ability against a vibration or impact from the outside become small.